yes, correct, but I never noticed such a hugh difference (15 fps) between both modes.
Maybe I should just wait for the next betas, I guess that there will be some more changes in the xplane code until it goes final.
The last resort would be upgrading my graphiccard to a newer one .
But beside this I'm very happy now with the Skymaxx pro !
P.s. there is a similar discussion in xplane.org:
https://forums.x-plane.org/index.php?/forums/topic/212074-clouds-under-vulcan-disproportionate-impact-on-frame-rate/